Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements, a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin or similar natural or synthetic hydrophilic polymers have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin or similar polymers are formulated so as to facilitate contact between the silver halide crystals and aqueous processing solutions, the resultant coatings are not as fingerprint and stain resistant as would be desirable, particularly in view of the handling or environment that imaged photographic elements commonly experience under various circumstances. Thus, fingerprints can permanently mark conventional photographic elements. They can be easily stained by common household products, such as foods or beverages, for example, coffee spills.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damages by water or aqueous solutions. A number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. More recently, U.S. Pat. No. 5,853,926 to Bohan et al. discloses a protective coating for a photographic element, involving the application of an aqueous coating comprising polymer particles and a soft polymer latex binder. This coating allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. Again, however, the hydrophobic polymer particles must be fused to form a protective coating that is continuous and water-impermeable.
U.S. Pat. No. 5,856,051 describes the use of hydrophobic particles with gelatin as the binder in an overcoat formulation. This invention demonstrated an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. The hydrophobic polymers exemplified in U.S. Pat. No. 5,856,051 include polyethylene have a melting temperature (Tm) of 55 to 200.degree. C., and are therefore capable of forming a water-resistant layer by fusing the layer at a temperature higher than the Tm of the polymer after the sample has been processed to generate the image. The coating solution is aqueous and can be incorporated in the manufacturing coating operation without any equipment modification. Again, fusing is required by the photofinishing laboratories to render the protective overcoat water-resistant. This patent discloses that the incorporation of water soluble polymers at 5 to 45% by weight based on the total dry laydown of the overcoat layer can improve the developability and dye formation rate of the imaging formation layer. During processing, the water soluble polymers are removed from the coating. The average molecular weight of the water-soluble polymers is between 1,000 and 200,000. The patent lists a wide variety of non-ionic, anionic or cationic water-soluble polymers, including polyacrylamides and poly(vinyl alcohol).
Applicants have found that hydrophilic polymers in photoprocessing solutions at concentrations greater than 0.01 weight percent can be foam stabilizers, thereby causing the solutions to foam. Depending on the mode and level of agitation in the photoprocessing solutions, the severity of this problem can inhibit or even shut down the operation of a photoprocessing lab. There are two main problems that can occur when a stable foam is formed: (1) the foam can cause an increase in the volume of the processing solutions, and (2) the presence of foam can inhibit the wetting of the photographic element as it enters the processing solution, thereby causing a non-uniformity in the upperscale density area.
Chemical antifoamers in the prior art can be classified as one of two distinct types: (1) defoamers that break up a foam, and (2) insoluble organic materials. The details of the way these materials act to defoam as well as a broad range of examples are given in "Defoaming," P. R. Garrett Ed., Surfactant Science Series, Vol. 45 (Marcel Dekker, N.Y. 1993).
Examples of the first type of antifoamer, defoamers that break up a foam, are alcohols such as butyl alcohol, octyl alcohol, and the like. A deficiency of these materials is that their antifoaming action is short term. That is, they are able to break the foam at the time of addition, but cannot prevent subsequent formation of foam. They need to be added continuously, therefore, resulting in relatively large quantities of these materials accumulating in the solution. Examples of the second type of antifoamer, insoluble organic materials, are silicone oil, paraffin oils and dispersions of organic oils with silica particles in water. Frequently, these materials are made more effective by adding hydrophobic silica particles. These materials are quite effective at relatively low concentrations and have a sustained antifoaming action. Also, these materials are non-volatile. A characteristic of these materials are that they are either two phases or three phases in the aqueous medium. Consequently, while these materials work well to control foaming, they have the inherent problem of low shelf stability. The silica particles, due to their high density, usually settle out, while silicone oils rise to the surface. They also can contaminate hardware by sticking or coating the surfaces. Photoprocessing solutions are expected to have stability between the time of manufacture and the time of use, which may be on the order of months.
Surfactants have been used in photoprocessing for various reasons. The use of surfactants in developing solutions for silver halide imaging elements have been disclosed mainly to aid wetting of the imaging element. Japanese Kokai JP08201994 (1994) discloses an aminoacid-type surfactant to improve wetting. U.S. Pat. No. 5,447,817 discloses an anionic surfactant in the developer to prevent "pi marks" for X-ray film. Japanese Kokai JP-06130581 discloses the use of a nonionic surfactant in the developer for a B&W silver halide imaging element, to improve processing uniformity. Surfactants required to improve wettability are typically used at relatively high concentrations in order to reduce the surface tension of the processing solution. Surfactants have also been used in developer solutions to reduce the formation of deposits or "tar." Japanese Kokai JP-05273712 and JP-05273711 disclose a nonionic surfactant in the developer solution to process an imaging element containing a fluorosurfactant in the antistat layer, to prevent scumming. Japanese Kokai JP-2915091 discloses the use of an ethoxylated surfactant in the developer solution containing sulfite, to minimize deposition of tar like material during processing. Surfactants have also been disclosed for use in developing solutions to minimize stain in the processed imaging elements. Japanese Kokai JP-06250360 discloses the use of a water soluble surfactant in the developer in order to reduce the replenishment rate in the stabilizer and still minimize stain. U.S. Pat. No. 5,091,292 discloses the use of anionic surfactants to reduce stain upon photoprocessing.
It would be desirable to improve the photoprocessing of photographic imaging elements having a nascent water-resistant overcoat. Furthermore, it would be desirable that to improve the photoprocessing of photographic imaging elements that have an overcoat formulation comprising at least one water-dispersible hydrophobic polymer interspersed with a water-soluble hydrophilic polymer that is leached into a photoprocessing solution.